1. Field of the Invention
The present invention relates to an image sensor structure and a method of fabricating the same, and particularly to an image sensor structure and a method of fabricating the same capable of avoiding or mitigating shade effect.
2. Description of the Prior Art
For digital imaging apparatuses, such as digital still cameras or digital video cameras, image quality is one of the most significant design issues. In an image generated by an image sensor of a conventional digital imaging apparatus, the central portion of the image is typically brighter than the peripheral portion of the image. This phenomenon is also referred to as lens shading effect or vignetting phenomenon, which is caused by a non-uniform photo-response across the lens of the digital imaging apparatus. It is found that if the chief ray angle is greater than 20 degrees, the brightness of the peripheral image is 78% or less of the brightness of the central image. Hence, various lens shading compensation methods have been disclosed in order to mitigate the lens shading effect.
FIG. 1 illustrates a cross-sectional diagram of a conventional CMOS image sensor structure. In the CMOS image sensor structure 10, photodiodes 12 are formed in the surface of a substrate 14. A passivation layer 22 covers each of three metal conductor layers 16, 18, and 20. A dielectric layer 24 covers each passivation layer 22 and is planarized. An under layer 26, which is planar and with a substantially uniform thickness, is formed on the up most dielectric layer 24. Color filters 28, such as red filters 30, green filters 32, and blue filters 34 are formed on the under layer 26. A top layer 27 is formed on the color filters (CFs) 28. Microlenses 36 are formed on the top layer 27. With such structure, focal length can be sufficient for allowing light to focus on the photodiodes 12 within the substrate 14, and, accordingly, the shading effect on such structure is not obvious.
However, with the demand for lighter, thinner, shorter, and smaller devices, the focal length must be shortened. Thus, the chief-ray angle is increased and the shading effect becomes significant. FIG. 2 illustrates a combination of a conventional CMOS image sensor structure and a lens module. Photodiodes 12 are located within the surface 15 of the substrate 14. All of the photodiodes, dielectric layers, or protection layers are not shown for purpose of concise and easy reading. As shown in FIG. 2, in the situation that the focal length is too short, after the incident light 38 passes through the lens module 40, a light beam reaches the edge, passes through the microlens 42 or 43 at the edge, and passes through the top layer 27, the color filter (CF) 28, the under layer 26 and the dielectric layer 46 to focus on the point A or A′; and another light beam reaches the center, passes through the microlens 44 at the center, and passes through the top layer 27, the color filter 28, the under layer 26 and the dielectric layer 46 to focus on the point B. It can be found that the points B and A or A′ are not on a same planar face. The B point is lower. The arc line 48 roughly depicts a line formed by all focus positions. In a plan view, the focus positions form a concave face having a depth difference d from the horizontal surface 15 of the substrate 14 where the photodiodes 12 are located, leading to non-uniform photo-response and making the peripheral image darker.
FIG. 3 illustrates a conventional way to mitigate shading effect by shifting the microlens inward and/or shifting the color filter outward, in order to make the focuses to be along with the positions of the photodiodes at the surface of the substrate to reduce the depth difference, that is, to make the depth d′ (for point B′) to become zero if possible. However, as shorter focal length is demanded, the chief-ray angle becomes greater, where the shifting of the microlenses or the color filters is spatially limited and can not well compensate the shading effect.
Therefore, there is still a need for a novel image sensor structure which is small, light, short, and thin, without suffering from shading effect.